1. Field of the Invention
The invention relates to solenoid operated valves for petroleum production wells and, more particularly, to a power supply and control arrangement for an electrical solenoid operated safety valve system.
2. History of the Prior Art
Oil and gas wells, and in particular those located offshore, are frequently subject to wellhead damage which may be produced by violent storms, collisions with ships and numerous other disastrous occurrences. Damage to the wellhead may result in the leakage of hydrocarbons into the atmosphere producing the possibility of both the spillage of the petroleum products into the environment as well as an explosion and fire resulting therefrom. In addition to off-shore production wells, another environment in which damage to a wellhead may have disastrous effects is that of producing wells located in urban areas. Moreover, in such urban production wells, it is generally a specific legal requirement that there be some downhole means of terminating the flow of petroleum products from the well in the event of damage to the wellhead. In such instances, the safety valve system must be responsive to a dramatic increase in flow rate from the well so as to close down and terminate production flow from the well. For these reasons, sub-surface safety valves located downhole within a borehole have long been included as an integral part of the operating equipment of a petroleum production well.
Various types of petroleum production flow safety valve systems have been provided in the prior art. Each system includes a valve means for controlling the flow of petroleum products up the tubing from a point down in the borehole from the wellhead. Safety valve systems also include sensing means which are responsive to wellhead damage, a dramatic increase in production flow, or some other emergency condition requiring that the flow from the well be terminated by the valve.
One type of operating mechanism used to actuate a safety valve within a well includes an electrical solenoid employed to hold the safety valve in an open condition and a spring means to return it to a normally closed condition in response to interruption in the flow of current to the solenoid. Numerous such systems have been proposed, for example, U.S. Pat. No. 4,002,202 to Huebsch et al, U.S. Pat. No. 4,161,215 to Bourne, Jr. et al, and U.S. Pat. No. 4,566,534 to Going III. Each of these systems provide a solenoid actuated operating mechanism for the safety valve which is responsive to a DC electric current supplied from surface equipment. Such solenoids generally require a fairly high level surge of initial operating current to cause the solenoid to operate and change states and then a smaller level of current to hold the solenoid in its operated condition. These large actuating current surges require heavy electrical conductors in order to carry such current downhole for any substantial distance and still maintain a voltage level sufficient to operate the solenoid. Such heavy electrical conductors are both relatively expensive as well as difficult to install and maintain in a downhole production flow environment.
In subsurface safety valve systems, it is also highly desirable to be able to monitor the state of actuation of a solenoid operated safety valve in order to be able to provide a positive indication to an operator at the surface as to the open/closed state of that valve in order to monitor and control the output from the well as a function of the valve position. Prior art systems for monitoring the condition of a solenoid operated safety valve have included U.S. Pat. No. 4,321,946 to Paulos et al. The Paulos system includes monitoring the voltages on the winding of the solenoid as the solenoid changes state from one condition to another. Paulos' circuitry monitors the back EMF generated by the solenoid as it changes states to indicate the fact that the safety valve has in fact changed state in response to an actuation voltage applied to it. One drawback of such systems is that an indication of the safety valve condition of operation is only produced during the brief time period when the solenoid is changing states. There is no way in which system relying upon the generation of back-EMF can be continuously monitored during the valve open condition to ensure that the valve remains in the open condition.
In other prior art systems, the conductive pipe comprising the tubing and casing of a petroleum production well have been used to conduct a flow of electricity from the surface down into the borehole. For example, in U.S. Pat. Nos. 3,507,330 and 3,642,066 to Gill, a flow of AC current is passed down a conductive path including the tubing and casing of a well and through the connate water within the geological formation itself in order to heat the oil based production fluids within the formation to reduce their viscosity and effectuate their flow into the producing well. Similarly, in U.S. Pat. No. 3,958,636 to Perkins, a pair of wells which are spaced transversely from one another in a formation have electrodes inserted down into the boreholes of the wells. Electric current is caused to flow between the two electrodes in order to heat the geological formation between the wells to increase the flow of the petroleum products from the wells. In virtually all of such prior art systems, current sent down the conductive tubing portions of a well completion is used to heat or treat the formation rather than to control electrically operated devices down in the well.
The inherent disadvantages of providing heavy expensive electrical cabling downhole is obviated by the system of the present invention which provides means for coupling current from the surface down the electrically conductive path formed by the well tubing and casing and interconnecting that current to the windings of a solenoid actuated safety valve. The system allows actuation of the solenoid and control of the safety valve in response to signals and power generated at the well head. In addition, the system of the present invention also allows continuous monitoring of the open/closed condition of the solenoid operated safety valve by measuring parameters of the solenoid continuously so that any change in the condition of the valve is indicated immediately to a monitoring station at the well head.
The system of the present invention overcomes many of the inherent disadvantages of the prior art electrically operated solenoid actuated safety valve systems as well as enables continuous monitoring of the state of such safety valves.